Current product trends in electronics are requiring flexible circuits of finer and finer pitch. A repeating defect caused by small particles that adhere to a phototool during the imaging step of the flexible circuit making process may significantly reduce the product yield.
The fabrication of flexible circuits involves the creation of several layers of dielectric and conductive materials that are in intimate contact with layers adjacent to them. At least one of these layers may be patterned by selectively introducing material into or removing material from that layer. The pattern may be created by photolithographic processes. For example, a layer of photoresist material can be applied onto the surface of the layer to be patterned. A phototool having transparent and opaque areas in the form of the desired pattern can be used to selectively expose the photoresist to ultraviolet light. The light will either cause portions of the photoresist to undergo a crosslinking reaction in the exposed areas, as in the case of a negative photoresist, or to undergo a polymeric degradation reaction in the exposed areas, as is the case with a positive photoresist. An appropriate solvent may be used to remove the desired portion of the photoresist. The exposed underlying area may be etched away in the case of subtractive processing or added to in the case of additive processing. In either case the layer is patterned.
Photolithographic processes enable the creation of flexible circuits having excellent feature resolution as well as allowing high throughput during the manufacturing process. If different patterns are applied to different layers, the phototool must be correctly aligned on the photoresist layer. The phototool may be secured to the photoresist by clamping or by pulling a vacuum when the phototool is placed in contact with the photoresist during this photolithographic process.
However, defects in the pattern or the phototool are routinely experienced, especially when the phototool is used repeatedly to print several substrates consecutively without cleaning the phototool. Consequently, phototools must be inspected and cleaned regularly. This can affect the throughput of the lithographic process as well as introduce added cost if the defects cannot be eliminated and the phototools must be replaced.
Conventional phototools often have chrome and glass regions. Light can pass through the glass regions but not the chrome regions. Both glass and chrome are high surface energy materials, which can cause particles of the photoresist or dust to adhere to the phototool. When particles stick to the glass, light is absorbed and, as a result, does not reach the photoresist. This can result in inadequate exposure of a given area, which in turn creates defects. Furthermore, particles that adhere to the phototool can create a gap between the phototool and the photoresist surface, reducing resolution of the resulting image.